Conventional motor vehicles are equipped with a power steering system, a so-called power assisted steering system, in order to reduce the steering effort for the driver. On the one hand, the power assisted steering system is used to facilitate handling in town traffic at low speeds of the motor vehicle, while, on the other hand, a direct steering sensation for the driver is made available at high speeds of the motor vehicle. Most motor vehicles use a hydraulic power assisted steering system (HPAS) for this purpose. However, electric power assisted steering systems (EPAS) are also known. Instead of a hydraulic fluid, they use an electric motor and a gear mechanism to make the power assistance available to the steering system. But so-called steer-by-wire systems in which the vehicle tires are decoupled mechanically from the steering wheel are also under development. These systems have an additional electric motor for generating feedback to the driver.
EP 1 183 176 B1 relates to a steering device for a motor vehicle which is configured to understeer, with steerable front wheels, with a steering element which can be activated by a driver counter to the effect of a steering force or of a steering torque, and with at least one actuator for the lock angle of the steerable wheels, which actuator acts on the steerable wheels and can be influenced by an electronic control device. A device is provided for determining a lateral slip occurring at the steerable wheels. An upper limiting value for the transverse slip can be determined in the electronic control device, with a means being provided which serves to counteract a further increase in the lock angle if the lateral slip which is determined is greater than the upper limiting value.
However, EP 1 183 176 B1 also relates to a steering method for the steering device. The steering element actuator makes the steering element vibrate if the lateral slip which is determined is greater than the upper limiting value. The steering method disclosed in EP 1 183 176 B1 is intended to be suitable in particular for steer-by-wire systems. However, it should also be capable of being used in power-assisted hydraulic or electric steering systems with a rigid mechanical connection between the steering element and steerable wheels.
DE 100 41 178 A1 relates to an operator control device for controlling a motor vehicle having an operator control element which can be moved in a number of degrees of freedom. The operator control element gives tactile feedback, and tactile feedback can be generated in all the degrees of freedom of activation by means of corresponding actuators for signaling in a perceptible way when borderline situations of the vehicle are reached or are imminent.
EP 0 856 453 A2 discloses an electrohydraulic steering system for vehicles which comprises a manual steering system and an automatic steering system (autopilot) which can be activated by means of a switch. The electrohydraulic steering system is composed of at least one hydraulic steering cylinder for adjusting the steerable wheels, at least one sensor for determining the respective wheel lock angle actual values, at least one hydraulic control valve which can be activated electrically and which controls the application of hydraulic fluid to the steering cylinder, at least one automatic steering signal transmitter for generating electric steering signal set point values for the wheel lock angles, an electronic open-loop control line devaluation device, with in each case the automatically generated steering signal set point values and the wheel lock angle actual values being fed to the open-loop control and evaluation device and with the open-loop control and evaluation device determining in each case an electrical activation signal for the hydraulic control valve, using the wheel lock angle actual value and the automatically generated steering signal set point value. A steering signal transmitter which generates from a manual actuating movement an electric steering signal set point value which corresponds to said movement is provided for the manual steering, with this steering signal set point value being supplied to an open-loop control and evaluation device which also receives the respective wheel lock angle actual values. The open-loop control and evaluation device determines an electrical activation signal for the hydraulic control valve, using the wheel lock angle actual value and the manually generated steering signal set point value.
U.S. Pat. No. 6,370,460 B1 discloses a steer-by-wire control system comprising a vehicle tire unit which responds to vehicle tire control signals for controlling the vehicle, a steering wheel unit which responds to steering torque control signals, a vehicle speed sensor for generating a vehicle speed signal, and a main control unit which is of electrical design and is connected to the vehicle speed sensor, the steering wheel unit and the vehicle tire unit. The vehicle tire unit also has a vehicle tire position sensor and a track rod sensor in order to generate and transmit a track rod force signal. In addition, the steering wheel unit has a steering wheel position sensor in order to generate and transmit a steering wheel position signal, and a force sensor in order to generate and transmit a feedback torque signal.
The U.S. Pat No. 6,370,460 B1 system main control unit has a position control unit which calculates the vehicle tire control signal in comparison with the speed signal, the feedback torque signal and the steering wheel position signal. Furthermore, the main control unit generates an opposed track rod force signal. In addition, the main control unit has a force control unit which calculates and generates the steering wheel force control signal in comparison with the opposed track rod force signal, the feedback torque signal, the steering wheel position signal and the speed signal.
U.S. Pat. No. 6,535,806 B2 also discloses a steer-by-wire system. It comprises a steering wheel, a steering wheel sensor which is arranged in relation to the steering wheel in order to both determine its position and to generate an indicating signal for the position of the steering wheel. The system also has a feedback control unit in order to make available an indicating signal for a current direction of the vehicle. A control unit is connected to the steering wheel sensor of the feedback control unit in order to receive the indicating signal for the position of the steering wheel from the steering wheel sensor and receive the indicating signal for the current direction of the vehicle from the feedback control unit. The control unit generates an indicating signal for a necessary steering wheel feedback torque. The system further includes a tactile feedback device which is connected to the control unit which receives the indicating signals for the necessary steering wheel feedback torque, the feedback device making a corresponding force available to the steering wheel.
It is considered a general disadvantage of the known motor vehicles with the hydraulic and electric power assisted steering systems or the steer-by-wire systems, that they are subject to specific service intervals and the abovementioned power steering systems may degrade in performance or at least be restricted in terms of their functionality, not only if the service intervals are not complied with. For this reason, it is appropriate to monitor this operator control unit, which is mentioned by way of example, so that a loss of performance can be communicated at an early point to the driver of the motor vehicle.
The communication is usually done in conventional motor vehicles by means of an audible display or visual display. However, in such motor vehicles the driver is confronted with an increasing number of audible and/or visual signals. This constitutes a heavy load of visual and/or audible messages, which distracts the attention of the driver from the traffic activity in order to perceive the respective display devices. In particular, the visual displays may be responsible for this, since they frequently lie outside the field of vision which is necessary to observe the road traffic.
For this reason a solution is desired based on the object of using simple means to improve a motor vehicle of the type mentioned at the beginning to the effect that the motor vehicle driver's attention is not disrupted if a warning signal is to be displayed to him.